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W. Zhao et al. / Catalysis Communications 11 (2010) 527–531
sample of boric acid were dissolved in 50 ml of boiling water. A
solution of 6 M hydrochloric acid was added (ꢀ19 ml) with vigor-
ous stirring in order to dissolve the local precipitate of tungstic acid
until the pH was 6. The solution was kept boiling for 1 h and after
cooling to room temperature it was kept 24 h at 4 °C. The precipi-
tate was removed by filtration, and then the potassium salt was
precipitated from the solution by addition of potassium chloride
(ꢀ10 g). The crude potassium salt was dissolved in 100 ml of luke-
warm water, the insoluble part eliminated, and the potassium salt
precipitated again by addition of KCl (ꢀ9.6 g). Anal. Calcd for
K8[BW11O39H]Á13H2O: K, 9.74; B, 0.36; W, 63.11; H2O, 0.06. Found:
K, 9.90; B, 0.32; W, 62.95; H2O, 0.07. IR spectrum (KBr, cmÀ1): 992,
954, 889, 841, 807, 751, 630, 514, 476, 436.
alcohol and 1-pentanol) were oxidized to aldehyde and acid. The
results listed in Table 1 show the linear aliphatic alcohols (for
example, 2-octanol and 1-pentanol) were much more difficult to
oxidize than that of cyclic alcohols. So a 5-fold molar excess of
H2O2 and 10 h reaction time were needed for the oxidation of these
linear alcohols (entries 6–8). However, for more active substrates
such as cyclohexanol (entries 1–3), 1-phenylethanol (entry 5)
and benzyl alcohol (entry 9), a 2- fold molar excess of H2O2 was
sufficient. For cyclohexanol, a conversion of 88% (99% selectivity)
(entry 2) was obtained within 1 h, which the molar ratio of
H2O2/cyclohexanol was 2. When the reaction time proceeded to
3 h, the conversion of cyclohexanol reached 99% (entry 3) with
the same high selectivity of cyclohexanone. Even when the amount
of H2O2 and substrate were equivalent, cyclohexanol was also oxi-
dized to cyclohexanone in 6 h with 93% yield (entry 1). Although
cyclohexanol is very active in this catalytic system, the control
experiment showed that no product was detected in oxidation of
cyclohexanol without adding catalyst (entry 4). The efficiency of
hydrogen peroxide utilization was determined by iodometric ana-
lyzing method at the end of each reaction. As shown in Table 1, the
H2O2 efficiency was obtained from 10% to 93% based on different
substrates. A decrease in H2O2 efficiency was observed when the
H2O2/substrate ratio increased. This result should be attributed
to the property of the substrate. Linear aliphatic alcohols, contain-
ing 2-octanol, 2-pentanol and 1-pentanol, were difficult to oxidize,
so a 5- fold molar excess of H2O2 and 10 h reaction time were
needed for the oxidation of these linear alcohols. Under the tem-
perature of 90 °C, the long reaction time would increase the
decomposition of hydrogen peroxide. Besides, the high molar ex-
cess of H2O2 and the low conversion of the substrate also made
that the H2O2 efficiency was lower in the calculation after the oxi-
dation of linear aliphatic alcohols. On the contrary, the low molar
of H2O2 and the high conversion of alcohol produced a high H2O2
efficiency (94%, entry 1).
2.3. Characterization techniques
Infrared spectra were recorded on a Nocolet FTIR-360 FT–IR
spectrometer. The catalysts were measured using 2–4% (w/w)
KBr pellets prepared by manual grinding. Chemical elemental anal-
ysis of the catalysts was done on an ICP-atomic emission spectrom-
eter (IRIS ER/S). GC analyses were performed on Shimadzu GC-9AM
with a flame ionization detector equipped with FFAP capillary col-
umn (internal diameter = 0.25 mm, length = 30 m). GC–MS was re-
corded on Finnigan Trace DSQ (Thermo Electron Corporation) at an
ionization voltage of 70 eV equipped with a DB-5 capillary column
(internal
diameter = 0.25 mm,
film
thickness = 0.25 lm,
length = 30 m). 1H NMR and 13C NMR spectra were recorded on a
Bruker AM-400 and Varian mercury 300 MHz spectrometer with
TMS as an internal standard and CDCl3 as solvent unless otherwise
noted.
2.4. Catalytic reaction
The catalytic reactions were performed in a 25 ml two-necked
round-bottomed flask equipped with a septum, a magnetic stirring
bar, and a reflux condenser. The oxidation was carried out as fol-
lows: catalyst (0.015 mmol), water (3 ml), substrate (1 mmol),
and H2O2 (30% aq.) were charged in the reaction flask. The reaction
was carried out at 363 K and detected by TLC accompanied with
GC. When the reaction was over, the organic products were sepa-
rated from the aqueous phase by extraction, and then the organic
layer was analyzed by GC with the internal standard method.
Assignments of products were made by comparison with authentic
samples. Selected products were also analyzed by GC/MS (Finnigan
Trace DSQ), 1H NMR and 13C NMR. After separation of product, re-
addition of alcohols (1 mmol) and H2O2 (30% aq.) to the aqueous
phase containing the catalyst was carried out for the next oxida-
tion cycle.
Primary alcohol was difficult to oxidize and the selectivity to
the target product of aldehyde or carboxylic acid was low. In this
oxidation system, 1-pentanol was oxidized to valeric acid and val-
eraldehyde with 19% and 18% yield, respectively (entry 8). How-
ever, it was active for oxidation of benzyl alcohol due to the
active phenyl group. Benzaldehyde was the main oxidation prod-
uct in this system with 98% conversion and 83% selectivity (entry
9).
It is well known that benzaldehyde is a very important chemical
which has widespread applications in the fields of flavors, odorants
and pharmaceutical intermediates [22]. In some catalytic systems,
it was difficult to control the producing of benzaldehyde in the oxi-
dation of benzyl alcohol, and often benzoic acid was obtained in-
stead of benzaldehyde. The catalyst of Na12[WZnZn2(H2O)2
(ZnW9O34)2] [18] oxidized primary alcohols to the corresponding
carboxylic acid, including benzyl alcohol. Another catalyst of Na6[-
SiW11ZnH2O40]Á12H2O [19] showed high activity in oxidation of
benzyl alcohol, but the product was also benzoic acid.
3. Result and discussion
3.1. Catalytic performance
The oxidations of benzyl alcohol catalyzed by K8[BW11
O39H]Á13H2O in water with different molar ratio of H2O2/benzyl
alcohol were investigated in detail and the results are shown in Ta-
ble 2. When the amount of H2O2 and benzyl alcohol was equivalent
(entry 2), the conversion of alcohol reached 95% with the products
containing benzaldehyde and benzoic acid, but the main product
It is advantage for practical application because the catalyst
K8[BW11O39H]Á13H2O is effective and recyclable for the oxidation
of various alcohols in water (Scheme 1). The solvent of water used
in this system is much cheaper and safer than any other organic
solvents. After the reaction finished, the organic products were
separated from the aqueous phase by simple extraction. The prod-
ucts separation procedure was much easier than the traditional
chromatographic procedure.
Table 1 summarizes the results of catalytic oxidation of various
alcohols in water with 30% H2O2 based on K8[BW11O39H]Á13H2O.
Secondary alcohols were oxidized to the corresponding ketones
in high conversion and selectivity. The primary alcohols (benzyl
R1
R2
R1
R2
K8[BW11O39H].13H2O
ΟΗ
Ο
water, H2O2, 90oC
Scheme 1.